M^The Circulatory System 



LARGE animals could not exist without an 

 effective circulation. Foods from the diges- 

 tive tract must be carried to all the cells of 

 the body, and the respiratory gases must be 

 transported between the various tissues and 

 the breathing organs. Metabolic wastes have 

 to be collected and brought to the excretory 

 organs; and hormones and other special sub- 

 stances must be distributed throughout the 

 body without delay. Diffusion and osmosis 

 alone are much too slow to fulfill these needs, 

 except in very small and simple animals such 

 as Hydra. Accordingly, all larger animals 

 have developed one or more circulatory 

 fluids, which are forcefully pumped through- 

 out the body for the transportation of neces- 

 sary substances. 



DIRECT CIRCULATIONS; HEMOLYMPH 



Some lower animals possess only one circu- 

 latory fluid, the hemolymph, which performs 

 the functions of both the blood and the 

 lymph of higher forms. Hemolymph fills the 

 tissue spaces, coming into contact with the 

 cells; and thus the hemolymph carries ma- 

 terials directly to the cells. 



Planaria and the other flatworms (p. 635) 



318 



possess a very simple hemolymph system. The 

 hemolymph of Planaria flows through the 

 intercellular spaces that permeate the loosely 

 packed mesodermal tissues. No heart is pres- 

 ent to pump the body fluid; but the hemo- 

 lymph flows irregularly back and forth 

 throughout the body, as a result of the mus- 

 cular movements of the worm. Such a slow 

 and irregular flow suffices to distribute foods 

 to the various tissues, because the gastrovas- 

 cular cavity of this animal sends branches to 

 all parts. Branches of the excretory system 

 also reach all the tissues, so that the meta- 

 bolic wastes are not carried very far before 

 they are eliminated. Nor is the hemolymph 

 very important in the respiration of Planaria. 

 The animal is relatively small, and an ex- 

 change of gases between the cells and the sur- 

 rounding water takes place quite rapidly. 



The hemolymph system of the lobster (p. 

 318) and other Arthropoda (p. 654) is some- 

 what more complex. The heart of the lobster 

 is a hollow muscular sac, which lies in a 

 pericardial cavity that is filled with hemo- 

 lymph (Fig. 17-1). When the heart expands, 

 it sucks in hemolymph through valved open- 

 ings, the ostia, which penetrate the cardiac 

 wall. Then, when the heart contracts, the 



